1. Field of the Invention
This invention concerns wood pulp made from hardwoods by the sulfite process and paper made from the pulp. More particularly, this invention concerns a specially prepared wood pulp which has unusual softness properties and the soft tissue paper webs made from the pulp.
2. Background Art
The sulfite pulping or digestion process and, more particularly, the acid sulfite pulping process which is sometimes referred to as the acid bisulfite process are well known and extensively practiced throughout the paper industry. Excellent discussions of the acid bisulfite pulping process, as well as of the operations and equipment used in conjunction with it, are found at numerous places in the literature. Three commonly cited references are: (a) McDonald, Ed. Pulp and Paper Manufacture: Volume 1: The Pulping of Wood (Second Edition), McGraw-Hill (New York, 1969); (b) Casey, Pulp and Paper Chemistry and Chemical Technology: Volume 1: Pulping and Bleaching, (Second Edition), Interscience publishers (New York, 1960); and (c) Rydholm, Pulping Processes, Interscience Publishers (New York, 1965).
Acid bisulfite processes are also disclosed in the patent literature such as U.S. Pat. No. 4,295,929 which issued to Leithen on Oct. 20, 1981.
All of the references listed above, which are incorporated herein by reference, and others of their ilk, provide the background and the framework for the present invention. The teachings of these references pertaining to process operations, equipment, process conditions, and the like can be used in conjunction with the present invention except insofar as the present invention requires modification of the teachings in unique and unobvious ways to produce the unexpected results obtained by the practice of the present invention.
Morgan and Rich in U.S. Pat. No. 3,994,771, issued Nov. 30, 1976, incorporated herein by reference, teach and claim a process for forming a wet-laid composite, soft, bulky, and absorbent paper web comprising at least two layers prepared from at least two furnishes preferably comprising different fiber types. This process has been used to make an exemplary, commercially successful sanitary tissue comprising three layers. The center layer comprises primarily northern softwood kraft pulp fibers while the two outer layers are each formed from a furnish comprising a mixture of shorter fibers, normally about 40% northern hardwood sulfite fibers and about 60% eucalyptus fibers. The presence of the shorter fibers in the two outer layers markedly increases the tactile feeling of softness of the sanitary tissue product. More especially, the softness is markedly enhanced by the use of eucalyptus fibers in combination with common sulfite hardwood fibers in the outer layers. While the product, constituted as described, is quite acceptable to consumers, eucalyptus fibers are more expensive than other more common fibers such as sulfite hardwood fibers and are sometimes subject to vagaries of supply. Replacement of the eucalyptus fibers with other less expensive and more readily available fibers is and has been a sought after goal, but one which is reasonable and acceptable only when the resulting eucalyptus-free product maintains the physical characteristics of the eucalyptus-containing product.